Magnetron beam switching tube



y 9, 1963 R. w. WOLFE 3,091,322

MAGNETRON BEAM SWITCHING TUBE Filed June 21, 1960 2 Sheets-Sheet z 23 1l ZERO -0 SET 4 9 43 FLIP vs l JNVENTOR.

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SOURCE ATTORNEY July 9, 1963 R. w. WOLFE MAGNETRON BEAM SWITCHING TUBEFiled June 21, 1960 157/? 52.0 '5 g 3 In TARGET POTENTIAL (uoLTs) F/G. 4

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Y Roam W. WOLFE G4 G4 64 MC /gm A T TOR/VEY 3,097,322 Patented July 9,1963 3,097,322 MAGNETRON BEAM SWITCHING TUBE Roger W. Wolfe, SouthPlainfield, NJ., assignmto Burroughs Corporation, Detroit, Mich, acorporation of Michigan Filed June 21, 1960, Ser. No. 37,607 13 Claims.(Cl. 313-156) This invention relates to multi-position magnetron beamswitching tubes and, particularly, to improved electrode arrangementsfor such tubes.

One type of multi-position beam switching tube operates with crossedelectric and magnetic fields and includes a cathode and a plurality ofgroups of electrodes surrounding the cathode, each group of electrodescomprising a position at which an electron beam may be formed and fromwhich an output signal may be derived. Each group of electrodes includesa target or output electrode which receives the electron beam andprovides an output signal therefrom, a spade electrode which forms andholds the electron beam on its associated target electrode, and aswitching electrode which is used to switch an electron beam from oneposition or group of electrodes to the next. A longitudinal magneticfield is used in operation of this type of tube, and the magnetic fieldmay be provided by a cylindrical magnet secured to and surrounding thetube envelope or by a plurality of magnet rods positioned within theelectrode assembly or in any other suitable manner.

One problem which has faced users of beam switching tubes results whenan attempt is made to draw unusually large target currents. Because ofthe fact that the targets have a pentode-type operating characteristicwhich is desirable in general, the potential of a target must bemaintained in a relatively narrow range of positive potentials forstable operation. If the potential of a target is reduced below thisrange of potentials, then the target is unstable and spurious switchingof an electron beam, from this target to the next, takes place. Thisinstability arises from the fact that a target, when lowered to acertain level of potential, is unable to collect all of the electrons inthe electron beam which flows to it. The excess electrons spread out,flow to adjacent electrodes, and ultimately cause spurious switching.Methods are known for overcoming this problem; however, such methodsrequire comparatively complicated and expensive circuitry.

Another problem in the use of the above-described beam switching tubesresults from the physical relationship of the target and switchingelectrodes in any group of electrodes. Since the target and switchingelectrodes are adjacent to each other electrically and the electricfield of one aiiects the other, the potential on a target electrode atany instant affects the operation of the switching electrode. Thus, theswitching characteristics of the tube are aliccted by the targetpotentials.

Accordingly, the principles and objects of the present invention areconcerned with the provision of an improved multi-position magnetronbeam switching tube in which the operating stability and the efiiciencyof the switching operation are generally independent of target voltage.

The purposes and objects of the invention are accomplished by means of anovel electrode arrangement in a magnetron beam switching tube. The tubeincludes a central cathode and a plurality of groups of electrodessurrounding the cathode. Each group of electrodes include a spadeelectrode which is closest to the cathode. The space between adjacentspades defines a path for an electron beam. A target or output electrodelies behind the spade with an unobstructed current flow path providedbetween each target and the cathode. Each group of electrodes alsoinclude a rod-shaped electrode and an auxiliary electrode which occupythe space between adjacent groups of electrodes. The rod-shapedelectrode may be used essentially as a switching electrode and theauxiliary electrode may be used essentially as a shield electrode whichmakes the switching electrode substantially independent of the targetelectrically.

The invention is described in greater detail by reference to thedrawings wherein:

FIG. 1 is a perspective view of a magnetron beam switching tubeembodying the invention;

FIG. 2 is a plan view of the electrode assembly of the tube of FIG. 1;

FIG. 3 is a schematic representation of the tube of FIG. 1 and a circuitin which it may be operated;

FIG. 4 is a graph illustrating the characteristics of one aspect of theoperation of a tube embodying the invention;

FIG. 5 is a graph illustrating the characteristics of another aspect ofthe operation of a tube embodying the invention;

FIG. 6 is a schematic representation of the tube of FIG. 1 and anothercircuit in which it may be used;

FIG. 7 is a schematic representation of the tube of FIG. 1 and stillanother circuit arrangement in which it may be used; and

FiG. 8 is a schematic representation of the tube of FIG. 1 and stillanother circuit arrangement in which it may be used.

Referring to FIGS. 1 and 2, the principles of the invention are embodiedin a magnetron beam switching tube 1!] which includes an envelope 12which contains an electrode assembly 14 comprising a centrallongitudinally elongated indirectly heated electron emitting cathode 16and ten groups of elongated electrodes spaced radially from andsurrounding the cathode.

Each group of electrodes represents a position at which an electron beammay be formed and from which a corresponding output signal may bederived. An electron beam may be switched from position to positionunder the influence of crossed electric and magnetic fields within theelectrode assembly. The direction in which an electron beam tends toswitch is determined by the orientation of these fields and may beclockwise or counter-clockwise. The electrode configuration shown isadapted for clockwise movement of an electron beam. The configurationmay be reversed for counter-clockwise movement. A position which isahead of an electron beam at any instant is known as a leading position,and its electrodes are leading electrodes. A position which is behindthe beam is known as a lagging position, and its electrodes are laggingelectrodes. In addition, the terms leading" and lagging are applied toappropriate portions of the individual electrodes in each group ofelectrodes.

According to the invention, each group of electrodes includes fourelectrodes rather than three as in prior art comercial magnetron beamswitching tubes. This adance provides added flexibility and utility inmagnetron beam switching tubes so that they are now able to performoperations which, heretofore, could not be performed satisfactorily.

Referring to FIGS. 1 and 2, each group of electrodes in the tube 10includes a spade electrode 18 which is substantialiy the same instructure and function as the spade electrode in prior art magnetronbeam switching tubes such as the type 6700 tube. Each spade electrodeserves to form and hold an electron beam on its associated targetelectrode. The spade electrodes are generally U-shaped and each includesa base portion 20 which is closest to the cathode and from which twoarms 22 and 24 extend substantially radially in a direction away fromthe cathode.

The spade electrodes are generally elongated and are disposed parallelto the cathode aligned on a circle having the center of the cathode 16as its center. The spade electrodes are the closest electrodes to thecathode and adjacent spades provide, between them, a space or paththrough which an electron beam may flow.

Each group of electrodes also includes a target or output electrode 26which is intended to be the primary receiver of an electron beam andfrom which an output signal is derived. In the embodiment of theinvention shown in FIGS. 1 and 2, the target electrode also performs thefunction of providing the required longitudinal magnetic field withinthe envelope. Accordingly, the target is a permanent magnet and ispreferably in the form of a rod. Any other suitable means for providinga longitudinal magnetic field may also be used. The targets are orientedparallel to the cathode and lie on a circle having the center of thecathode as its center. The target rods are more remote from the cathodethan the spade electrodes 18 and are positioned to receive an electronbeam flowing from the cathode. In the embodiment of FIG. 1, each targetrod is positioned substantially between the arms of the associated spadeelectrode but closer to the leading arm so that, if the leading arm wereextended radially, it would strike the target rod at about the mostleading portion of its surface or somewhat in the lagging directionthereof. The target is thus radially aligned with its spade and ispositioned favorably to receive an electron beam without beingundesirably close to a. rod-like switching grid electrode 28 which isalso provided in each group of electrodes.

The switching grids 28 are parallel to the cathode and lie on a circlehaving the center of the cathode as its center. Each switching grid issubstantially radially aligned with the lagging arm of the adjacentleading spade electrode and is immediately adjacent to that arm. In anyone group of electrodes, the switching grid associated with that groupof electrodes is at the leading side of the group.

According to the invention, each group of electrodes also includes agenerally L-shaped auxiliary electrode 30 which, in the tube 10, iscalled a shield grid because one of its important functions is to shieldeach target electrode 26 from the associated switching electrode 23. Theauxiliary electrode 30 is positioned between and occupies the spacebetween each target 26 and its switching grid 28 and thus serves toelectrically isolate the latter two electrodes from each other. Theresultant advantages of this construction are described below. Theauxiliary electrode is shaped and positioned so that it performs itsfunction without obstructing the electron flow path between the cathodeand a target electrode.

The L-shaped auxiliary electrode 30 includes a first generally radialarm 32 which includes a leading end 34 and a trailing end 36. The radialarm 32 is substantially aligned with the lagging arm 22 of the adjacentleading spade 18, and the leading end 34 of the radial arm is closelyadjacent to the switching electrode 28 which lies between it and thelagging arm of the leading spade. The auxiliary electrode 30 alsoincludes a transverse arm 38 which defines the radial extent or boundaryof each group of electrodes. The transverse arm 38 extendsperpendicularly from the trailing end 36 of the radial arm 32 andextends, in a lagging direction, to close to the target rod 26 withwhich it is associated. The transverse arm thus closes the space betweenadjacent target rods 26.

One suitable circuit for operating the tube 10 is shown in FIG. 3. Thetube is shown schematically in linear form with only five groups ofelectrodes or beam positions illustrated. In the circuit, the cathode 14is connected to ground. Each of the spade electrodes 18 is connectedthrough a suitable spade resistor 40 to a common spade bus bar 42 whichis suitably coupled by lead 44 to a positive DC. power supply V of about250 volts. A suitable zero-set circuit for setting an electron beam atthe position after the tube has been cleared is provided. This may besimply a switch 45 connected between the 0 spade and ground. Each targetrod electrode 26 is connected through a suitable target load resistor 46to a target bus bar 48 which is coupled to a suitable positive DC. powersupply V of about 300 volts. Each target is also provided with aterminal 50 which may be connected to any suitable utilization device,for example, an indicator tube, a printing mechanism, or the like. Theauxiliary electrodes 30 are connected together and are connected by lead51 to the spade bus 42 whereby the positive spade bus potential isapplied thereto. The switching grid electrodes 28 are connected in twosets, with the electrodes at the even-numbered positions in one set andthe electrodes at the odd-numbered positions in the other set. Each setof switching electrodes is connected to one of the output terminals of asuitable flip-flop 56. A source 58 of suitable switching pulses iscoupled to the input of the flip-flop. The switching electrodes arebiased at a small positive voltage, for example, about 25 volts in anysuitable manner. This bias may be applied through the flip-flop circuit.

In operation of the circuit of FIG. 3, each input pulse from the source58 to the flip-flop 56 provides an output pulse which is coupled to oneof the sets of switching grids which thereby causes the electron beam inthe tube 10 to be switched by one position. As an electron beam flows toa position, most of the electrons in the beam are collected by thetarget rod electrode under most operating conditions and normal, stabletube operation results. This condition prevails if the potential of eachtarget electrode is maintained at a sufficiently high level so that itis able to collect substantially all of the current in the electronbeam. With this type of stable operation, the auxiliary electrodes 30perform no significant function. However, under circumstances where thetarget electrode is reduced to a considerably low potential, such ascathode potential, a target may not be able to collect all of theelectrons which are available to it. This represents a condition ofinstability and ordinarily would cause spurious switching of theelectron beam to the next leading position. However, the auxiliaryelectrode 30 associated with the target electrode which is receiving anelectron beam is able to collect any electrons which cannot be collectedby the target so that stability is maintained. Thus, the target may bereduced to substantially any potential without adversely affectingstable operation of the tube.

The significance of the auxiliary electrode 30, also known as a shieldgrid, may be clearly illustrated by reference to FIG. 4 in which curve Ishows the variation in target current with target potential and curve Ishows the variation in shield grid current with target potential. Inprior art magnetron beam switching tubes, the stability of the tubedepends to a considerable extent upon the level of target voltage. Atlow target voltages (voltages below the knee of the targetcharacteristic curve I the tube tends toward instability because all ofthe available current cannot be collected by a target and some currentleaks to other tube elements. This current leakage is able, if notcontrolled, to cause spurious switching of an electron beam from oneposition to the next. FIG. 4 illustrates how, according to theinvention, this leakage current is collected in tube 10 by the shieldgrid 30 as the target voltage falls below the knee of its characteristiccurve. It is this function of the shield grid that makes operation oftubes embodying the invention substantially independent of targetvoltage. Previously, operation below the knee of the targetcharacteristic curve was possible to a limited extent by means ofcomparatively complex circuit arrangement; however, with the addition ofthe shield grid, operation without such circuitry and with targetpotentials as low as zero volts is possible.

The above-described function of the auxiliary electrodes 30 has solved aproblem which often disturbed users of beam switching tubes in the past.It has been customary, in many applications, to couple each target of abeam switching tube to one of the cathodes of a cold cathode gaseousindicator device such as the 6844A indicator tube. Such a tube has acharacteristic gas ionization time, which accompanies cathode glow, andthis time may be of the order of two to five microseconds. During thistime interval, the indicator tube represents an extremely highimpedance. Thus, there is a strong tendency for the targets to bottom,that is. to drop to cathode potential. If it were not for the shieldgrid, this bottoming would result in excessive current leakage andspurious switching. However, as illustrated by FIG. 4, during this shorttime interval when the target has bottom, the shield grid 30 effectivelyabsorbs the excess leakage current and prevents spurious switching dueto instability. Finally, when the indicator tube fires, the targetpotential rises to a level at which it can collect the electrons in abeam. This new operating characteristic of the tube of the invention isequally useful in the operation of all non-linear devices, such as gasdischarge tubes, relays and pulse transformers.

Improved switching characteristics are closely associated with theimproved target characteristics described above. Practice and theoryshow that the position of a target electrode with respect to itsassociated switching electrode and the respective electrical potentialsof these electrodes affect the eificiency of operation of the switchingelectrode in performing a beam switching operation. It has been found inprior art beam switching tubes that the amplitude of the switching pulseapplied to the switching electrodes from the flipfiop to cause a beam toswitch positions is dependent, in general, upon the potential of thetarget to which a beam is flowing at any instant. This means that, asthe target voltage is increased a larger switching pulse amplitude isrequired to effect proper switching action. This results essentiallyfrom the fact that a more dense, more compact electron beam isassociated with high target potentials. Conversely, as the targetpotential is lowered, the electron beam becomes less compact, leakagecurrents increase, and a smaller amplitude switching pulse is requiredto elfect switching.

However, in the tube embodying the invention, each switching electrode2.8 primarily sees the potential of the auxiliary electrode 30 and isthus primarily influenced thereby. The switching electrodes are,accordingly, substantially unaffected by target potential. This aspectof the invention is illustrated in FIG. in which curves A and B relateswitching voltage and target voltage at different switching gridpositive bias voltages, for example, approximately 25 and 15 volts,respectively. The curves show that, for a particular bias voltage,substantially the same magnitude of switching pulse will cause a beam toswitch although the target voltage may vary over a relatively widerange.

It is also known that the potential of the target electrode, in general,has its electrical effect on the switching electrode and thus has itsinfluence on the switching operation. This is particularly evident athigher frequencies of tube operation. In normal operation of a beamswitching tube, when an electron beam is switched and flows to a targetelectrode, the potential of the target is reduced to some low level.Later, when a switching pulse is applied to the associated switchingelectrode to cause the beam to switch to the next position, theswitching operation is aided by this relatively low potential on thetarget. However, as the speed of operation of the tube is increased, theresistance-capacitance time constant of the target circuits prevent atanget from dropping to this favorable low potential when it receives abeam before the next switching pulse is applied. The faster theoperation, the higher the target potential remains. Thus, since thetarget is at an undesirably high potential at the time the switchingoperation is to occur, the switching operation is made more dilficultand a larger driving pulse is required.

However, in the tube embodying the invention, since each switchingelectrode always sees primarily the poten- 6 tial of the auxiliaryelectrode 30, it is substantially unaffected by target potential. Thus,the tube 10 is characterized by uniform operation independent of thefrequency of operation.

In addition to the usual operations for which tube 10 may be employed,the auxiliary electrodes 30 may be utilized in many diflierent ways. Forexample, referring to FIG. 6, which is assumed to include all of thenecessary connections shown in FIG. 3, a source of negative switchingpulses 60 is shown coupled to the auxiliary electrodes 30. The source ofswitching pulses is adapt-ed to apply a negative pulse to the auxiliaryelectrode associated with the target electrode which is receiving anelectron beam at the same time that a switching pulse is applied to theassociated switching electrode to cause the electron beam to switch tothe next position. A negative pulse applied to an auxiliary electrode inthis manner promotes a condition of instability of the type describedand aids the switching operation.

In FIG. 7, each of the auxiliary electrodes 30 is provided with anoutput terminal 62 which may be used to obtain an output signal when anelectron beam is switched from one position to the next. The outputpulse thus provided may be used, for example, to drive a subsequent tubein a cascade counter or for any other suitable purpose.

In FIG. 8, each auxiliary electrode 30 is electrically connected to thelagging spade electrode immediately adjacent thereto by a. lead 64. Thisconnection causes a negative pulse to be applied to the lagging spadeelectrode during a beam switching operation. This arrangement impartsdesirable characteristics to the electron beam and improves tubeoperation.

The principles and advantages of the invention are clearly illustratedin the foregoing discussion of the invention. The requirements of thevarious electrodes and the tube in general are clear, and those skilledin the art will appreciate that various modifications might be made inthe tube structure and in the associated circuitry within the scope ofthe invention.

What is claimed is:

l. A magnetron beam switching tube of the type utilizing crossedelectric and magnetic fields to control the flow of an electron beam andoperating to switch an electron beam in a direction determined by theorientation of said fields and known as the leading direction,

said tube having an electrode assembly including a cathode for providinga stream of electrons and a plurality of groups of electrodessurrounding the cathode,

each group of electrodes defining an electron beam receiving region andincluding a target electrode for receiving an electron beam andproviding an output signal therefrom,

a spade electrode lying between said cathode and the target electrodefor forming and holding an electron beam thereon, said spade and targetlying generally at one side of said electron beam receiving region,

a switching electrode spaced from said target and spade electrodes andlying generally at the other side, the leading side, of said beamreceiving region,

and an auxiliary electrode lying outside of the current flow path fromcathode to target and substantially completely enclosing the spacebetween said target electrode on one side and both said switchingelectrode on the other side and the target electrode of the next leadinggroup,

the auxiliary electrode thus being positioned to collect leakage currentfrom an electron beam flowing to a target and to provide a favorableelectric field configunation to render a target able to receive an electron beam over a wide range of target potentials.

2. The tube defined in claim 1 wherein said electrode assembly includespermanent magnet providing a longitudinal magnetic field in said tube.

3. The tube defined in claim 1 wherein each auxiliary electrode isprovided with an output terminal whereby it may be selectively employedas an output electrode which receives an electron beam.

4. The tube defined in claim 1 wherein the electrode means in each groupof electrodes is electrically connected to the adjacent lagging spadeelectrode.

5. The tube defined in claim 1 wherein one electrode in each group ofelectrodes is a permanent magnet.

6. The tube defined in claim 1 wherein each target electrode is apermanent magnet.

7. The tube defined in claim 1 wherein all of the tube electrodes arelongitudinally elongated and are parallel to each other and wherein,further, each spade electrode has a generally U-shaped cross sectionwith the base of the U facing the cathode and the open portion of the Ufacing away" from the cathode, each target electrode disposed at leastin part in the open end of its associated spade electrode, there beingan unobstructed current fiow path between each target and the tubecathode.

8. A magnetron beam switching tube including an electrode assemblyincluding a cathode for providing a stream of electrons; and a pluralityof groups of electrodes surrounding the cathode; each group ofelectrodes including a target electrode for receiving an electron beamand providing an output signal therefrom, a spade electrode lyingbetween said cathode and the target electrode for forming and holding anelectron beam thereon, said spade and target electrodes beingsubstantially in radial alignment, a switching electrode spaced fromsaid target and spade electrodes, and an auxiliary electrode for themost part more remote from said cathode than said spade and the electroncollecting area of said target and essentially lying across andoccupying substantially all of the space between a target electrode onone side and both its associated switching electrode and the adjacentleading target electrode, said auxiliary electrode being accessible toelectrons from an electron beam flow of an electron beam to the targetelectrode.

9. The tube defined in claim 8 wherein each spade electrode is generallyU-shaped in cross-section and is oriented with the open portion thereoffacing away from the tube cathode,

each target electrode is in the form of a rod and is positioned in theopen end of and close to the associated spade electrode and accessibleto electrons from the cathode,

each switching electrode lies closely adjacent to the adjacent spadeelectrode, and

each auxiliary electrode is generally L-shaped in crosssection andincludes a first arm which extends from close to its target electrodeacross the space between adjacent target electrodes to close to the nextleading target electrode,

the auxiliary electrode includes a second arm which lies closelyadjacent to the leading target electrode and.

extends substantially radially to terminate close to its switchingelectrode,

but not obstructing the normal,

the auxiliary electrode being thus adapted to receive leakage currentfrom an electron beam and to render its target operable in normalfashion to receive an electron beam over a relatively wide range ofoperating target voltages.

10-. The tube defined in claim 8 wherein each auxiliary electrode isprovided with an output terminal.

11. The tube defined in claim 8 wherein each auxiliary electrode iselectrically connected to the adjacent lagging spade electrode.

12. The tube defined in claim 8 wherein said auxiliary electrodesinclude means for applying switching pulses thereto.

13. An electron tube for operation with crossed magnetic and electricfields comprising an elongated envelope,

an elongated cathode centrally disposed therein,

an array of similar elongated spade electrodes disposed adjacent to saidcathode and adapted to form and hold an electron beam,

each of said spades having a trough-shaped transverse cross-section, thesides of said spades extending generally away from said cathode,

an array of similar elongated target electrodes disposed adjacent tosaid array of spades and on the side thereof which is more remote fromsaid cathode,

each spade being substantially radially aligned with its target and saidcathode,

an array of elongated auxiliary electrodes, each lying adjacent to oneof said targets on the leading side thereof,

each of said auxiliary electrodes having a beam receiving surface facingsaid cathode and one side member angularly disposed therewith andextending radially inwardly toward said cathode,

the beam receiving surface of each auxiliary electrode being disposed inthe space between adjacent target electrodes and the side member of eachauxiliary electrode extending radially inwardly toward the lagging sideof the adjacent leading spade electrode,

and an array of switching electrodes in the form of rods each positionedbetween the side member of its auxiliary electrode and the side memberof the adjacent leading spade electrode,

each group of electrodes thus provided being adapted to form an electronbeam on its target electrode with the auxiliary electrode being adaptedto receive leakage current not collected by its target electrode wherebytube operation is stabilized and optimum control of an electron beam isachieved.

References Cited in the file of this patent UNITED STATES PATENTS

1. A MAGNETRON BEAM SWITCHING TUBE OF THE TYPE UTILIZING CROSSEDELECTRIC AND MAGNETIC FIELD TO CONTROL THE FLOW OF AN ELECTRON BEAM ANDOPERATING TO SWITCH AN ELECTRON BEAM IN A DIRECTION DETERMINED BY THEORIENTATION OF SAID FIELDS AND KNOWN AS THE LEADING DIRECTION, SAID TUBEHAVING AN ELECTRODE ASSEMBLY INCLUDING A CATHODE FOR PROVIDING A STREAMOF ELECTRONS AND A PLURALITY OF GROUPS OF ELECTRODES SURROUNDING THECATHODE, EACH GROUP OF ELECTRODES DEFINING AN ELECTRON BEAM RECEIVINGREGION AND INCLUDING A TARGET ELECTRODE FOR RECEIVING AN ELECTRON BEAMAND PROVIDING AN OUTPUT SIGNAL THEREFROM, A SPADE ELECTRODE LYINGBETWEEN SAID CATHODE AND THE TARGET ELECTRODE FOR FORMING AND HOLDING ANELECTRON BEAM THEREON, SAID SPADE AND TARGET LYING GENERALLY AT ONE SIDEOF SAID ELECTRON BEAM RECEIVING REGION, A SWITCHING ELECTRODE SPACEDFROM SAID TARGET AND SPADE ELECTRODES AND LYING GENERALLY AT THE OTHERSIDE, THE LEADING SIDE, OF SAID BEAM RECEIVING REGION, AND AN AUXILIARYELECTRODE LYING OUTSIDE OF THE CURRENT FLOW PATH FROM CATHODE TO TARGETAND SUBSTANTIALLY COMPLETELY ENCLOSING THE SPACE BETWEEN SAID TARGETELECTRODE ON ONE SIDE AND BOTH SAID SWITCHING ELECTRODE ON THE OTHERSIDE AND THE TARGET ELECTRODE OF THE NEXT LEADING GROUP, THE AUXILIARYELECTRODE THUS BEING POSITIONED TO COLLECTED LEAKAGE CURRENT FROM ANELECTRON BEAM FLOWING TO A TARGET AND TO PROVIDE A FAVORABLE ELECTRICFIELD CONFIGURATION TO RENDER A TARGET ABLE TO RECEIVE AN ELECTRON BEAMOVER A WIDE RANGE OF TARGET POTENTIALS.